add av_cold to vp9dsp.c initializers.
VP9 MC (ssse3) optimizations.

Decoding time of ped1080p.webm goes from 20.7sec to 11.3sec.
Full-pixel MC functions.

Decoding time of ped1080p.webm goes from 11.3sec to 11.1sec.
Use UTF-8 for my name in copyrights.
Use av_assert*() instead of assert().